Hierarchically structured mobile communication networks are inter alia used for voice communication and recently more and more for data communication, for example between two mobile stations or between a mobile station and the internet, for receiving and sending of E-mails or the like. A hierarchically structured mobile communication network may include one or more base stations that provide services to a certain coverage area. Due to the hierarchically structure of the mobile communication network a macro cell base station covers a plurality of small cell base stations within the macro cell base station coverage area. A small cell base station within the macro cell base station coverage area may be connected to a mobile communication core network or the macro cell base station for providing services. The base stations are part of a more complex radio access network with a plurality of differently located macro cell base stations and small cell base stations preferably with at least partially overlapping coverage areas to provide uninterruptable services when changing the coverage area. Macro cell base stations and small cell base stations may be directly connected to each other and also have a connection to the mobile communication core network providing further services. Small cell base stations have in general smaller coverage areas than macro cell base stations and may even be installed by customers of an operator providing services.
In order to save energy the current draft standard of IEEE 802.16m according to IEEE, “IEEE P802.16m/D8, Part 16: Air Interface for Broadband Wireless Access Systems, Advanced Air Interface” of August 2010 supports a so called low-duty-mode (M. Einhaus, L. Fan, T. Ikeda, A. Maeder, J. Zhou, Y. Zhou “Method for operating a WiMAX femtocell base station and femtocell base station”, PCT/EP 2009/007670) for reduced energy consumption and for reduction of interferences between different small cell base stations and/or macro cell base stations. In the low-duty-mode the base station preferably avoids air interface transmissions including in particular a transmission of frame preambles, pilots and broadcast channels if there is no operational need, for example if there is no mobile station within the coverage area of the base station.
However, if a mobile station enters the coverage area of a small cell base station which is currently in the low-duty-mode an air interface of the small cell base station is offline although the low-duty-mode allows a basic default transmission pattern to provide at least visibility via the air interface to mobile stations scanning for small cell base station reference signals, in particular for frame preambles of radio frames. This only provides a very slow connectivity between the mobile station and the small cell base station in the low-duty-mode, since the reference signals are broadcasted in relatively long time intervals, so that the mobile station needs time to identify the reference signal of the small cell base station and then to transmit a corresponding signal to activate the small cell base station. After the wake-up or activation of the base station the connection has to be established between the mobile station and the small cell base station in an active operational mode. This is a very time consuming procedure.
In addition to the above mentioned low-duty-mode, in PCT/EP2010/002636 a radio interface listen mode and a radio interface off mode is defined, so wake-up conditions of the small cell base station and corresponding procedures may vary according to the current suboperational mode of the base station. For example a small cell base station in the radio interface off mode cannot be activated according to the same procedure as a small cell base station in the low-duty-mode. Another problem is that a switching from an inactive operational mode to a more active operational mode of the base station may not be required, if only a transmission of a low traffic volume is requested or if a hand-over to a small cell base station may decrease a quality of service of the active connection between the mobile station and the base station, so that a direct connection between the mobile station and a small cell base station is not required or may not be beneficial and a connection between a macro cell base station and a mobile station may be sufficient in these cases.